The present invention relates to a method and a device for precooling the helium tank of a cryostat, in particular an optical cryostat comprising optical components in the helium tank, an NMR cryostat, a medical NMR cryostat for nuclear spin tomography for accommodating a superconductive magnet coil.
Cryostats accommodating a superconductive magnet coil have been previously known, for example in the field of nuclear magnetic resonance spectrometry or nuclear spin tomography (DE-A1-29 06 060, DE-B2-37 24 562). Such a cryostat comprises several tanks nested in each other, the innermost of them housing a magnetic coil and being filled in operation with liquid helium at a temperature of approx. 4 K. An outer tank contains liquid nitrogen at a temperature of approx. 77 K. Both tanks are vacuum-insulated relative to each other and from the surrounding temperature. The evacuated spaces which contain radiation shields are maintained at intermediate temperatures so that both heat transmission and heat radiation are reduced to a minimum. Before the first startup and also following all maintenance or repair work carried out on the magnetic coil or the cryostat, the latter must be cooled down to operating temperatures. Different methods of cooling down the cryostat to such operating temperatures have been known from the prior art. According to one of these methods it is proposed to fill the nitrogen tank with liquid nitrogen and the helium tank with liquid helium, and to cool it down in this way. However, such a method would require very important quantities of liquid helium as the magnetic coil and the inner space of the cryostat would have to be cooled down solely by the liquified helium. However, in spite of its low temperature of 4 K liquid helium has a considerably smaller capacity to absorb heat than liquid nitrogen at a temperature of 77 K. This lower heat-absorption capacity would lead to a considerable input of time, not to speak of the important consumption of liquid helium which, besides, is very expensive. These disadvantages cannot be justified by the argument that in order to avoid contaminations the helium tank is brought into contact in this manner exclusively with helium as a cooling medium, and this the less when the tank contains elements of important heat capacity, as for example a superconductive solenoid coil.
According to another proposed method, the evacuated spaces between the nitrogen and the helium tanks are cooled down by dry nitrogen gas so far that the helium tank is cooled to a temperature of 77 K by thermal conduction. This method is, however, connected with the disadvantage that the nitrogen has to be removed subsequently from the evacuated space. However, since this normally can be achieved only imperfectly, the nitrogen residues remaining in the evacuated space will freeze down on the colder helium tank when the system is put into operation subsequently. In addition, possible errors in operation of the vacuum valves present an increased safety risk as an overpressure may build up due to vaporization of condensed nitrogen and/or air, which may create an explosion risk. In addition, such indirect cooling via the evacuated space between the nitrogen tank and the helium tank is ineffective, time-consuming and will in addition lead to icing of the outer jacket of the cryostat in cases where only a single space exists between the nitrogen tank and the helium tank on the one hand and the environment on the other hand.
According to another--the most commonly used--method it is, therefore, proposed to fill the helium tank initially with liquid nitrogen whereby the tank is cooled down to a temperature of 77 K. As a result of this cooling step, most of the total heat is withdrawn from the cryostat. On the other hand, however, the liquid nitrogen has to be removed completely from the helium tank after the cooling step. Any nitrogen residues remaining in the tank would reduce the service life of the helium, which is normally in the range of one year, and in particular any nitrogen residues remaining on the magnetic coil would impair the operating safety of the magnetic coil, i.e. increase the risk of a quench, i.e. of an unwanted transition from the superconductivity to the normal-conductivity state of the magnetic coil. In the case of an optical cryostat, there would further be the risk of a coat of solid nitrogen forming on the optical components located in the path of the rays, such as windows, mirrors or the like. If the nitrogen contains a certain portion of oxygen, i.e. if the nitrogen is contaminated with oxygen, then this paramagnetic component will be attracted by the magnet coil when the latter is started up. In addition, the conversion work necessary for filling in and/or exchanging the liquified gases provides the increased risk that air from the atmosphere and/or humidity may condense into the system, and this may likewise lead to faulty operation and impair the service life of the helium. Moreover, there is an increased safety risk, due to an increased risk of operating errors.